1. Field of the Invention
The present invention relates to methods in the field of digital imaging, and more particularly, to methods for digital rendering of skin tissue or like materials to provide a more natural or lifelike appearance.
2. Description of Related Art
In recent decades, computers have increasingly been used to generate and/or modify moving images for virtually every application that makes use of them, for example, television and motion pictures, computer games, and engineering model building. As the processing power of computers has become increasingly affordable, more attention has been given to digitally representing all manner of scenes in a realistic, lifelike manner. The benefits of realistic digital representations should be readily apparent for entertainment and like applications, such as motion pictures and computer games. Realistic digital representation may also be quite useful in a number of non-entertainment applications, such as for training, cartography, medical imaging, or for any application that uses physical modeling. In general, realistic digital representations may be beneficial for any application in which it is desirable to simulate the appearance of actual physical objects, both moving and static, using a model that can be digitally manipulated. It is believed that use of digital modeling will become much more widespread in the future.
Numerous unsolved problems are recognized in field of realistic digital imaging. Many problems are associated with the digital representation of humans (or other animal forms, real or imaginary) in a realistic, lifelike manner. It is desirable in such cases that the use of the digital representation be undetectable by an attentive audience. Audiences are extremely perceptive of and sensitive to qualities associated with the human form, and especially of the human face and other skin areas under diverse lighting conditions. For many applications, therefore, it is desirable to realistically represent exposed areas of skin, and especially the face.
However, realistic rendering of skin tissue has been difficult to achieve due to the lack of efficient algorithms. Rendering algorithms are either excessively complex and time-consuming, or result in a waxy, plastic or lifeless result. Presently, painstaking effort by visual effects artists skilled in digital rendering may be required to realistically represent scenes involving faces or other exposed skin areas.
Automatic approaches to rendering may lessen the need for visual effects artists by automatically calculating color and intensity values, using an algorithm. In general, the interaction of skin tissue with light can be quite complex, making lifelike rendering of skin difficult, or at least, resource-intensive. It is known that the difficulty in rendering skin arises in part from the translucent nature of skin tissue, and subsurface scattering and transmission that occurs naturally in skin tissue. While mathematical models for accurately rendering translucent materials such as skin tissue do exist, these models are computationally complex. For example, methods employing the bi-directional reflectance distribution function (BRDF), the bi-directional surface scattering distribution function (BSSRDF) or Monte Carlo ray tracing are all computationally intensive. Computational requirements, therefore, make these prior art methods undesirable or unsuitable for rendering applications that require a relatively high volume and/or rate of rendering, such as motion picture, television, and other video production, and real-time applications such as video games. In view of the long-recognized complexity arising from the interaction of light with skin tissue, an efficient, compact algorithm for achieving lifelike rendering of skin would be both useful, and surprising.